Steam generator with helically coiled heat transfer tubes (HCT) has excellent compactness, high heat transfer coefficient and high thermal stress adaptability. In the existing studies on two-phase flow in HCT, the working fluids are mainly the combination of air/water, whose properties are significantly different from those of vapor/water. In this paper, the vapor/water bubbly flow in HCT was numerical investigated based on a VOF model, which was verified by comparing with the air/water experiments. The characteristics of interface distribution were obtained at pressures of 11–15 MPa and liquid superficial velocities of 1.2–3.2 m/s. The results showed that the vapor phase distributed in the inner and upper part of the tube due to the centrifugal force and buoyancy. However, because of the different physical properties of working fluid, the bubble clusters were located differently than in an air/water bubbly flow. As the pressure (temperature) increased, the vapor phase became more dispersed and the bubble chord length got shorter, due to the decrease of liquid surface tension. The bubble chord length also decreased with the increase of liquid velocity due to the enhanced buoyancy and shearing. In addition, the phenomenon of bubble aggregation in the flow direction was observed.